Polarization sensitive LiDAR system

ABSTRACT

A light detection and ranging (LiDAR) system includes light emitters that emit beams of light of substantially equal intensities. The light emitters form a beam polarization pattern with beams having varying polarizations. The LiDAR system also will include a receiver to receive light reflected from the object. An analyzer will determine characteristic differences between the beam polarization pattern of the beams emitted toward the object and an intensity pattern of the light reflected from the object, determine a reflection position that is associated with the light reflected from the object, and use the determined characteristic differences to determine whether the reflection position is a position of the object or a position of a ghost.

BACKGROUND

Light detecting and ranging (LiDAR) systems are used in variousapplications. One application for LiDAR systems is autonomous vehicles.Autonomous vehicles may use LiDAR systems to measure the distance fromthe autonomous vehicle to surrounding objects. To accomplish this task,the LiDAR system illuminates an object with light and measures thereflected light with a sensor. The reflected light is used to determinefeatures of the object that reflected it and to determine the distancethe object is from the autonomous vehicle. LiDAR systems also may beused in other applications, such as in aircraft, ships, mapping systems,and others.

An issue with many LiDAR systems is that light can be reflected frommultiple surfaces and/or objects, creating appearances of objects at thewrong location. For example, as illustrated in FIG. 1, light may beemitted from an emitter of a LiDAR system 101 toward a surface of anobject 102 which in this example is the hood of a vehicle. The light maybe reflected from that surface to another object 103 (in this case apipe in a ceiling), back to the surface of the first object 102, andthen to the receiving sensor of the LiDAR system 101. When this occurs,as shown in FIG. 1, it creates the appearance to the LiDAR of the object103 being in a different location 104. In other words, it creates whatmay be referred to as a “ghost”. These ghosts result in the LiDAR systembeing less reliable.

This document describes a polarization sensitive LiDAR system and methodthat is directed to solving the issue described above, and/or otherissues.

SUMMARY

In various embodiments, a light detection and ranging (LiDAR) systemincludes a group of light emitters that are configured to emit aplurality of beams of light. The light emitters are configured to form abeam polarization pattern and are positioned to be emitted toward anobject external to the LiDAR system. The LiDAR system also will includea receiver to receive light reflected from the object. The LiDAR systemalso will include an analyzer that has a processor and programminginstructions that are configured to cause the processor to determinecharacteristic differences between the beam polarization pattern of thebeams emitted toward the object and an intensity pattern of the lightreflected from the object, determine a reflection position that isassociated with the light reflected from the object, and use thedetermined characteristic differences to determine whether thereflection position is a position of the object or a position of aghost.

In various embodiments, each of the light emitters may be configured toemit a beam of polarized light that exhibits a polarization. Forexample, each of the light emitters may include a laser emitter, andeach beam of polarized light may be a polarized laser beam.

In various embodiments, the LiDAR system may include a polarizationmodifier that is configured to modify the polarization of at least aportion of the plurality of beams to create the beam polarizationpattern. For example, the polarization modifier may include half-waveplates or quarter-wave plates, each of which is positioned in a path ofa beam emitted by one of the light emitters. Alternatively, thepolarization modifier may comprise filters or mirrors, each of which ispositioned in a path of a beam emitted by one of the light emitters. Thehalf-wave plates, quarter-wave plates, filters or mirrors also may bepositioned in the paths for some but not all of the light emitters.

In some embodiments, a first portion of the light emitters may bemounted in a first orientation within a housing of the LiDAR system. Asecond portion of light emitters may be mounted within the housing in asecond orientation that is different from the first orientation, such asbeing perpendicular to each other (as in a relativelyvertical/horizontal orientation). Optionally, the first portion and thesecond portion may include equal numbers of light emitters.

In some embodiments, the receiver may include a polarization beamsplitter configured to split received light into vertical and horizontalpolarizations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates how a LiDAR system may detect a ghost in addition to(or instead of) a real object.

FIG. 2 illustrates example elements of a LiDAR system.

FIG. 3A illustrates an example of emitter laser beams with interleavedpolarization states.

FIG. 3B shows a possible pattern of received laser beams.

FIG. 4 illustrates how a waveplate may operate as a polarizationmodifier for light.

FIG. 5 illustrates an example of a diode laser with a waveplate thatserves as a polarization modifier.

FIG. 6A shows an example LiDAR system in which light emitter chips aremounted in alternating orientations. In comparison, FIG. 6B shows aconventional arrangement in which all emitter chips are positioned toemit light having a single, common polarization.

FIG. 7 illustrates an embodiment in which a set of beam-steering mirrorsserves as polarization modifier.

FIG. 8 illustrates an example method of using beam polarization patternsto distinguish a real object from a “ghost” in light received via aLiDAR system.

DETAILED DESCRIPTION

As used in this document, the singular forms “a,” “an,” and “the”include plural references unless the context clearly dictates otherwise.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. As used in this document, the term “comprising” (or“comprises”) means “including (or includes), but not limited to.” Whenused in this document, the term “exemplary” is intended to mean “by wayof example” and is not intended to indicate that a particular exemplaryitem is preferred or required. As used in this document, the term“light” means electromagnetic radiation associated with opticalfrequencies, e.g., ultraviolet, visible, infrared and terahertzradiation. Example emitters of light include laser emitters and otherdevices that emit light. In this document, the term “emitter” will beused to refer to an emitter of light, such as a laser emitter that emitsinfrared light.

In this document, when terms such “first” and “second” are used tomodify a noun, such use is simply intended to distinguish one item fromanother, and is not intended to require a sequential order unlessspecifically stated. In addition, terms of relative position such as“vertical” and “horizontal”, or “front” and “rear”, when used, areintended to be relative to each other and need not be absolute, and onlyrefer to one possible position of the device associated with those termsdepending on the device's orientation. This, when two items are referredto as being generally “vertical” and “horizontal” they are notnecessarily so oriented with respect to ground unless specificallystated, but instead are oriented perpendicularly with respect to eachother.

The terms “processor” and “processing device” refer to a hardwarecomponent of an electronic device that is configured to executeprogramming instructions. Except where specifically stated otherwise,the singular terms “processor” and “processing device” are intended toinclude both single-processing device embodiments and embodiments inwhich multiple processing devices together or collectively perform aprocess.

The terms “memory,” “memory device,” “data store,” “data storagefacility” and the like each refer to a non-transitory device on whichcomputer-readable data, programming instructions or both are stored.Except where specifically stated otherwise, the terms “memory,” “memorydevice,” “data store,” “data storage facility” and the like are intendedto include single device embodiments, embodiments in which multiplememory devices together or collectively store a set of data orinstructions, as well as individual sectors within such devices.

The present disclosure generally relates to a polarization sensitiveLiDAR system. References to various embodiments and examples set forthin this specification do not limit the scope of the disclosure andmerely set forth some of the many possible embodiments of the appendedclaims.

FIG. 2 shows an example LiDAR system 201 as may be used in variousembodiments. As shown in FIG. 2, the LiDAR system 201 includes a housing205 which may be rotatable 360° about a central axis such as hub or axle218. The housing may include an emitter/receiver aperture 211 made of amaterial transparent to light. Although the example shown in FIG. 2 hasa single aperture, in various embodiments, multiple apertures foremitting and/or receiving light may be provided. Either way, the systemcan emit light through one or more of the aperture(s) 211 and receivereflected light back toward one or more of the aperture(s) 211 as thehousing 205 rotates around the internal components. In an alternativeembodiment, the outer shell of housing 205 may be a stationary dome, atleast partially made of a material that is transparent to light, withrotatable components inside of the housing 205.

Inside the rotating shell or stationary dome is a light emitter system204 that is configured and positioned to generate and emit pulses oflight through the aperture 211 or through the transparent dome of thehousing 205 via one or more laser emitter chips or other light emittingdevices. The emitter system 204 may include any number of individualemitters, including for example 8 emitters, 64 emitters or 128 emitters.The emitters may emit light of substantially the same intensity, or ofvarying intensities. The individual beams emitted by 204 will have awell-defined state of polarization that is not the same across theentire array. As an example, some beams may have vertical polarizationand other beams may have horizontal polarization. FIG. 3A shows anexample of arrangement of emitted laser beams where odd beams havevertical polarization 301 and even number of beams have horizontalpolarization 302. Other patterns are also possible. Other states ofpolarization, such as left hand circular or right hand circularpolarization, are also possible.

Returning to FIG. 2, the LiDAR system will also include a light detector208 containing a photodetector or array of photodetectors positioned andconfigured to receive light reflected back into the system. The emittersystem 204 and detector 208 would rotate with the rotating shell, orthey would rotate inside the stationary dome of the housing 205. One ormore optical element structures 209 may be positioned in front of thelight emitting unit 204 and/or the detector 208 to serve as one or morelenses or waveplates that focus and direct light that is passed throughthe optical element structure 209.

FIG. 3B shows an example pattern of received laser beams. Between beamsnumber 9 and 16 the alternating intensity pattern shows that these beamsreflected off a tilted surface with a moderate angle of incidence. Beamsbetween 30 and 38 show a stronger modulated alternating pattern, whichis due to reflection of another surface with larger angle of incidence.Optionally, the detector (208 of FIG. 2) may be equipped with opticalelements that will separate the received light into two orthogonalstates of polarization, such as vertical and horizontal polarization.Then a detector can detect each state of polarization (such as thatshown in FIG. 3B), and the number of detectors will be the twice thenumber of emitters. For example, a system with 32 laser beams would beequipped with 64 receiver channels.

Returning again to FIG. 2, the LiDAR system will include a power unit221 to power the light emitter unit 204, a motor 223 that can turn theaxle 218, the housing 205 or other components, and electroniccomponents. The LiDAR system will also include an analyzer 215 withelements such as a processor 222 and non-transitory computer-readablememory 223 containing programming instructions that are configured toenable the system to receive data collected by the light detector unit,analyze it to measure characteristics of the light received, andgenerate information that a connected system can use to make decisionsabout operating in an environment from which the data was collected.Optionally, the analyzer 215 may be integral with the LiDAR system 201as shown, or some or all of it may be external to the LiDAR system andcommunicatively connected to the LiDAR system via a wired or wirelesscommunication network or link. For example, the motor 223 may beintegral with the LiDAR system, but the processor 222 and/or memory 223may be remote from the other components.

As noted above, one structure from which the emitter system may emitmultiple laser beams includes devices such as emitter chips, in whicheach emitter chip emits light from the single system. Additionally,although most lasers emit polarized light, in some embodiments the lightbeams may be unpolarized. With unpolarized light, the electric fieldvectors of the light will vibrate in some or all planes that areperpendicular to the direction of propagation of the light. In contrast,the electric field vectors of polarized light will vibrate in a limitedplane or direction. Polarized light exists in several different forms,some of the forms being linearly polarized in a single plane, othersbeing radially polarized (i.e., circularly polarized or ellipticallypolarized) in a single direction of rotation (e.g., right-handed orleft-handed). The use of polarized light beams in a LiDAR system givesthe LiDAR system a known polarization to use for an analysis whencomparing the emitted light to the received light. Most of the energy inbackscattered light will have a polarization that is directly related tothe polarization of the emitted light. In particular, if the transmittedlight is linearly polarized, the backscattered radiation will beexpected to primarily retain the polarization of the transmitted light.However, if the emitted light is circularly polarized, then thepolarization of the backscattered radiation will be expected to bereversed, i.e., right handed circular will be converted into left handedcircular and vice versa. Therefore, if the polarization of the receivedlight substantially matches the expected polarization, or if adepolarization ratio of the received light is near zero (in which thedenominator of the ratio is the backscatter intensity of the portion ofthe backscatter's orientation that matches the polarization of theemitted light), the LiDAR system may consider the received light to be areflection of the emitted light. As described below in the context ofFIG. 8, the system may use the intensity pattern of the received lightas a proxy to assess whether or not the polarization patterns match.

To help reduce the problem of “ghosts” described in the Backgroundsection of this document, the LiDAR system may also include apolarization modifier. The polarization modifier will be positioned andconfigured to alter the polarization of some, but not all, light beamsemitted by the system. As illustrated in FIG. 4, the polarizationmodifier may include any number of waveplates 402 that receives lightexhibiting a first polarization 401 and alters the light so that itexhibits a different polarization 403. Example waveplates include ahalf-wave plate that changes the polarization direction of linearlypolarized light (as shown in FIG. 4), or a quarter-wave plate thatconverts linearly polarized light into circularly polarized light orelliptically polarized light.

Such a waveplate may be positioned on or within the output of each lightemitter to which the waveplates are attached. For example, as shown inFIG. 5, if a diode laser 501 is the light emitter, light emitted from alight-emitting diode 502 may exit the emitter's housing 503 via a lens504. The lens 504 may include the polarization modifier 505 (which maybe a waveplate) either integrally or placed over or under the lensstructure, to alter the polarization of the laser beam as it exits thehousing 503. The waveplates may be placed over some portion, such ashalf, of the emitters, while the remaining emitters may have nowaveplates. Alternatively, two types of waveplates may be used so thatone portion (e.g., half) of the emitters emit light of a firstpolarization while the remaining portion (e.g., the other half) of theemitters emit light of a second (different) polarization.

By way of example, if the polarization modifier consists of half-waveplates modifier are used with half of the light emitters, the emittedbeams may exhibit a beam polarization pattern that contains halfvertically polarized laser beams and half horizontally polarized lightbeams.

In addition or alternatively, the polarization modifier may includequarter-wave plates which alter the polarization state of the lightbeams by introducing a phase shift. This phase shift may be used toconvert linearly polarized light to circularly or elliptically polarizedlight, or vice versa. For example, mounting a quarter-wave waveplate toa laser emitter that emits vertically polarized laser beams results incircularly polarized or elliptically polarized laser beams depending onthe orientation of the quarter-wave plate. The orientation of thequarter-wave plate can be varied to obtain right-handed circularly,left-handed circularly, right-handed elliptically, or left-handedelliptically polarized light. Additionally, right-handed circularly,left-handed circularly, right-handed elliptically, or left-handedelliptically polarized light may be obtained by using a combination ofquarter-wave waveplates and half-wave waveplates.

Alternatively, or in addition, since laser emitters each may emit apolarized laser beam, changing the orientation of the way some of thelaser emitters are mounted to the overall light emitter system canchange the polarization of those laser beams. For example, in someembodiments, laser emitters may emit vertically polarized laser beamswhen mounted to the laser unit in a first orientation. Changing themounted orientation of the laser emitters to an orientation that isperpendicular to the first orientation may result in the laser emittersemitting horizontally polarized laser beams. In this way, the laseremitters may be configured to emit multiple laser beams that have thesame polarization, or multiple laser beams that have one or moredifferent polarizations. Corresponding mounting arrangements may be usedwith other types of light emitters as well. For example, FIG. 6A showsan example LiDAR system 600 in which light emitter chips are mounted inalternating orientations, such that light emitter chips 601 and 603 arepositioned to emit light having a horizontal polarization, while lightemitter chips 602 and 604 are positioned to emit light having a verticalpolarization. Thus, each emitter chip that is positioned to emit lightof a first polarization (vertical or horizontal) will be positionedadjacent to at least one emitter chip that is positioned to emit lightof the other polarization. By way of comparison, FIG. 6B shows aconventional LiDAR system 610 in which all emitter chips 611-613 arepositioned to emit light having a single, common polarization.

Since light reflection alters the polarization of the light, one or morebeam-steering mirrors may be mounted in front of a light emitter tochange the polarization of a portion of the light beams. For example,referring to FIG. 7, mounting one or more mirrors 705, 706 to or infront of a laser emitter 701 that emits vertically polarized laser beams702 reflect the beams and result in horizontally polarized laser beams704 being emitted toward an object. Therefore, similar to the half-wavewaveplates, the mirrors 704, 706 may be used in place of changing themounting orientation of a portion of the laser emitters, or they may beused in conjunction with changing the mounting orientation of a portionof the laser emitters. In some embodiments, all of the laser emittersmay be mounted to the laser unit in the same orientation. The mirrors ofthe polarization modifier may be mounted to or positioned in front of aportion of the laser emitters, resulting in a polarization beam patternthat contains a portion of vertically polarized laser beams and aportion of horizontally polarized laser beams.

Regardless of the structure of the polarization modifiers used, they maybe used to impart a 90° rotation (or other modification) to thepolarization of the light emitted by the emitters to which they areattached. The polarization modifiers may be applied to the lightemitters in any determined pattern, such as every other emitter (e.g.,“even” emitters have a polarization modifier while “odd” emitters donot, or vice versa), in groups (e.g., two emitters with polarizationmodifiers followed by two emitters without), or in other patterns. Thepattern of application of polarization modifiers may vary, for exampleapplied only to emitters in a lower segment of the array (from whoselasers are more likely to generate ghost reflections).

The above examples described static (i.e., physically fixed)polarization modifiers. In other embodiments, the polarization modifiersmay include one or more optical elements that operate in dynamic manner.

Knowledge of the pattern will help the system distinguish a real objectfrom a “ghost” image. Referring to FIG. 8, the system will emit a groupof light beams toward an object in which each of the beams exhibits aknown intensity (which may be the same across all beams, or which mayvary across beams), and the beams' polarizations will vary, resulting ina known polarization pattern (step 801). The beams will be reflectedfrom that object, and the backscattered light will be received by theLiDAR system's receiver (step 802). Unlike that of the emitted beams,the intensities of the reflected and received beams will vary, and thereceived light will thus exhibit a polarization modified intensitypattern. The LiDAR system's processor may execute programminginstructions that cause the system's analyzer to analyze the intensitypattern of the received light (step 803) and determine characteristicdifferences (step 804) between the polarization pattern of the lightemitted toward the object and the intensity pattern of the receivedlight. When the emitted light beams reflect from an object, the receivedbeams' intensity pattern will not correspond to the polarization patternof the emitted beams.

For example, if emitted light includes only horizontally and verticallypolarized beams, the light reflected from the object may be rotated 90degrees before being received by the receiver. If the system receiveslight with this difference in its polarization orientationcharacteristic, it may presume that the light reflected from the object.In the same example, if the horizontally and vertically polarized beamsare reflected from a surface (typically one with a large angle ofincidence) to an object and back to the surface (e.g., from the hood ofa vehicle, to an object, and then back to the hood of the vehicle),which is the case for a ghost, the light will be received in anorientation similar to or the same as the orientation of the emittedlight. Thus, when determining the characteristic difference, in thisexample the lack of substantial difference (or in other words, thesubstantial similarity) may indicate that the reflection is a ghostrather than an object. In this document, the term “characteristicdifference” is intended to include both situations where the systemdetermines whether the beams' characteristics differ, as well assituations where the system looks for similarity (i.e., lack ofdifference) in the beams' characteristics.

Additionally, by using the time from emitting the beams of light toreceiving the reflected beams of light, the processor and programminginstructions may be used to determine the reflection position that isassociated with the light reflected from the object. Knowing that areceived beam intensity pattern that is the same as the emittedpolarization pattern represents reflection from a ghost, the processorof the analyzer uses the determined characteristic differences todetermine whether the reflection position is a position of the object ora position of a ghost (step 805). If the intensity pattern of thereceived light corresponds to the polarization pattern of the emittedlight, the processor will conclude that the reflection is from a ghost.Otherwise, if the intensity pattern of the received light corresponds tothe polarization pattern of the emitted light (as shown, for example, inFIGS. 3A and 3B) the processor will conclude that the reflection is fromthe object.

In some embodiments, the receiver of the LiDAR system (such as detector208 of FIG. 2) may include a polarization beam splitter. A polarizationbeam splitter is an optical device that splits a beam of light. Thepolarization beam splitter may be used to separate light, such as twoforms of light that have perpendicular orientations (e.g., verticallyand horizontally polarized light), which helps determine whether thereceived light has the same polarization as the transmitted light orwhether the polarization has been affected by reflections from varioussurfaces.

For example, in some embodiments, the receiver has one or more receiverchannels. Each receiver channel may be configured to detect one type ofpolarized light, such as vertically polarized light. So for a receiverto detect vertically polarized light and horizontally polarized light,at least two receiver channels are needed. The receiver can convert thereceived light to an electrical signal, and the strength of theelectrical signal may be used to determine if the received light wasmostly vertically polarized or mostly horizontally polarized. Theanalyzer may then use this information to determine characteristicdifferences between the emitted beam polarization pattern and the lightreflected from the object.

The features and functions described above, as well as alternatives, maybe combined into many other different systems or applications. Variousalternatives, modifications, variations or improvements may be made bythose skilled in the art, each of which is also intended to beencompassed by the disclosed embodiments.

The invention claimed is:
 1. A method of using a LiDAR system todifferentiate a ghost from a real object, the method comprising: by aLiDAR system, emitting a plurality of beams of light toward an objectthat is external to the LiDAR system, wherein the plurality of emittedbeams exhibit varying polarizations, which results in a polarizationpattern; by a receiver of the LiDAR system, receiving reflected lightthat exhibits varying polarizations, which results in an intensitypattern; and by a processor, executing programming instructions that areconfigured to cause the processor to: determine characteristicdifferences between the polarization pattern and the intensity pattern,determine a reflection position that is associated with the reflectedlight, and use the determined characteristic differences to determinewhether the reflection position is a position of the object or aposition of a ghost, wherein using the determined characteristicdifferences to determine whether the reflection position is a positionof the object or a position of a ghost comprises: if the polarizationpattern of the emitted beams substantially corresponds to the intensitypattern of the reflected light, determining that the reflection positionis a position of a ghost; and if the polarization pattern of the emittedbeams does not substantially correspond to the intensity pattern of thereflected light, determining that the reflection position is a positionof the object.
 2. The method of claim 1, wherein: receiving thereflected light comprises using a polarization beam splitter to splitreceived beams of light into vertically polarized light and horizontallypolarized light; and the method further comprises comparing thepolarization pattern of the emitted light with an intensity pattern ofthe beams received and split by the beam splitter to determine whetheran intensity pattern of the beams received substantially differs fromthe polarization pattern of the emitted light.
 3. The method of claim 1,wherein emitting the plurality of beams toward the object comprisesusing a polarization modifier to modify polarization of a portion of,but not all of, the plurality of emitted beams to create thepolarization pattern.
 4. The method of claim 3, wherein using thepolarization modifier comprises using a plurality of half-wave plates,each of which is positioned in a path of one of the emitted beams oflight, and the half-wave plates are positioned in the paths of some, butnot all, of the emitted beams of light.
 5. The method of claim 3,wherein using the polarization modifier comprises using a plurality ofquarter-wave plates, each of which is positioned in a path of one of theemitted beams of light, and the quarter-wave plates are positioned inthe paths of some, but not all, of the emitted beams of light.
 6. Themethod of claim 3, wherein using the polarization modifier comprisesusing a plurality of filters or mirrors, each of which is positioned ina path of one of the emitted beams of light, and the filters or mirrorsare positioned in the paths of some, but not all, of the emitted beamsof light.